This invention relates generally to controllers for lamps used to illuminate liquid crystal displays ("backlights") and the like and, in particular, to a backlight controller that accurately controls the apparent brightness of the backlight to the human eye.
Liquid Crystal Displays (LCD) provide a rugged and flexible display suitable for use in automotive applications. The LCD is backlit typically by a cold cathode fluorescent lamp (CCFL). Such fluorescent lamps are bright and relatively efficient and can be fabricated to provide even illumination over a large area.
Unfortunately, CCFL's are sensitive to temperature and vary in brightness as the passenger compartment and console warms up. One method of correcting for this variation is to make a temperature measurement at the CCFL and to vary its driving power to maintain constant illumination. The relationship between driving power, illumination and temperature is a complex function which may be implemented in lookup tables or algorithms incorporated into a microcontroller providing an output to the lamp. This approach is not entirely successful at low brightness levels and elevated temperatures where small changes in power, typically less than the quantization errors of the algorithms or tables, can result in greatly varying brightness.
A second approach to controlling the brightness of such lamps is to monitor their light output using an electronic light sensor and to use the measured output to control the power to the lamp by means of a feedback loop. A silicon photodiode may be used to measure the light from the lamp. Silicon photodiodes are inexpensive, rugged and available in a wide range of different package types including those suitable for surface mounting on a printed wiring board. Surface mount packages are smaller than packages that require sockets or holes in the printing wiring board to receive leads. Surface mounting lowers the cost of the photodiode.
Silicon photodiodes have sensitivity to a range of light frequencies that extend significantly into the infrared region invisible to the human observer. Unfortunately at cold temperatures, the CCFL's output a large amount of infrared radiation. For this reason, if an ordinary silicon photodiode is used to control a CCFL, the perceived brightness of the CCFL to a human observer will vary, being dimmer at cold temperatures, for example, when the output of infrared radiation is greater.
This problem may be solved by the use of an infrared filter, such as a special glass that absorbs the infrared portion of the light before it strikes the photodiode. Photodiodes with such filters, however, are relatively expensive and are not available in packages desired for use in the automotive field.